Need help with FTFs in a H&R 939

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My friend has an old H&R 939 that his grandfather gave him that is failing to fire about half of the rounds. It has been cleaned and oiled but we are still getting light indents on the primer and i think the hammer spring is bad. Does anyone know we can get a new spring and how hard it would be to replace it?

My friend has an old H&R 939 that his grandfather gave him that is failing to fire about half of the rounds. It has been cleaned and oiled but we are still getting light indents on the primer and i think the hammer spring is bad. Does anyone know we can get a new spring and how hard it would be to replace it?

Thanks DevilsJohnson, I had found that site and diagram as well however im not sure which parts i need to ensure that the hammer is hitting correctly. I am not that familiar with the inner workings of a firearm so any help in this matter would be greatly appreciated.